This invention relates to, in an optical pickup device provided with a plurality of light sources for emitting laser light beams different in wavelength with light emission points not existing on an identical optical axis, a method for receiving a reflected beam from a record side of an optical record medium by a common light receiving element, and relates to an optical pickup device using the light receiving method.
An optical pickup device of multiple light source type, provided with a plurality of laser light sources for emitting laser light beams different in wavelength is known as an optical pickup device for recording and reproducing a signal on a plurality of optical record media different in thickness, track pitch, etc. For example, a laser light beam having a wavelength of 780 nm is required for reproducing CD-R and thus an optical pickup device for reproducing CD-R as well as DVD and CD is of two-light-source type comprising a laser light source having a wavelength of 650 nm and a laser light source having a wavelength of 780 nm.
As such an optical pickup device of two-light-source type, the following configuration is known: As shown in a conceptual drawing of FIG. 5A, a monolithic two-wavelength semiconductor laser chip having a configuration in which light emission points of a red laser and a near-infrared laser, 101 and 102, are built in an active layer of a single semiconductor chip is used, and return light beams emitted from the semiconductor laser chip and then reflected on a reflection face of an optical record medium are diffracted by a hologram element, whereby the light receiving points of the return light beams of both wavelengths are matched and the return light beams are received by a common light receiving element and a signal can be detected.
In this case, the light emission points 101 and 102 for emitting laser light beams L1 and L2 of different wavelengths formed on the semiconductor laser chip 100 are 30 to 300 xcexcm apart from each other. Therefore, the laser light beams emitted from both the light emission points 101 and 102 are not positioned on the same axis. Thus, as shown in FIG. 5B, the optical axes of return light beams L1r and L2r of the wavelengths reflected on an optical record medium (not shown) and incident on a hologram element 103 are also apart at a similar distance from each other.
In the related art, both the return light beams L1r and L2r are diffracted by the hologram element 103 as primary diffracted light and are introduced into a common light receiving element 104. Generally, the wavelength xcex1 of the return light beam L1r is 650 nm and the wavelength xcex2 of the return light beam L2r is 780 nm. When a grating pitch of the hologram element 103 is defined as d, diffraction angles xcex81 and xcex82 of the primary diffracted light beams L1r(+1) and L2r(+1) are represented as follows:             θ      1        ≅          D      h        ≅                  λ        1            d                  θ      2        ≅                  D        +        Δ            h        ≅                  λ        2            d      
Thus, the following equations can be obtained.   D  =                              λ          1                                      λ            2                    -                      λ            1                              ⁢      Δ        ⁢          xe2x80x83        =          5      ⁢              xe2x80x83            ⁢      Δ      
Therefore, the positions at which the primary diffracted light beams L1r(+1) and L2r(+1) can be received by the common light receiving element 104 are positions apart a distance D about five times a light emission point spacing xcex94 in a direction perpendicular to the optical axis relative to the return light beams incident on the hologram element 103.
The described optical pickup device introduces the primary diffracted light beam of each wavelength into the common light receiving element. Therefore, the diffraction angle is comparatively large and if the wavelength of the laser light source changes with the environmental temperature, change in the light receiving position on the common light receiving element caused by change in the diffraction angle occurring accordingly is large. If the light receiving position on the common light receiving element changes, an offset easily occurs in a focusing error signal and a tracking error signal. When an offset occurs, an error correction cannot appropriately be made and thus there is a probability of degradation of the recording and reproduction performance of the optical pickup device.
It is therefore an object of the invention to provide a light receiving method capable of suppressing change in a light receiving position on a common light receiving element in an optical pickup device of multiple light source type and an optical pickup device using the light receiving method.
In order to achieve the above object, according to the present invention, there is provided a light receiving method in an optical pickup device, comprising the steps of:
providing a first light source and a second light source, light emission points of which are not on an identical optical axis;
providing a common light receiving element;
emitting a first laser light beam and a second laser light beam respectively from the first and second light sources, wavelengths of which are different from each other;
converging the first and second laser light beams onto a record face of an optical recording medium; and
diffracting the first and second laser light beams reflected from the optical recording medium based on different diffraction order, such that the diffracted first and second laser light beams are received by the common light receiving element.
If the return laser light beams different in wavelength are diffracted based on different diffraction orders, the diffraction angle of the diffracted light beam of each wavelength can be lessened and thus the light receiving position shift on the common light receiving element caused by wavelength change of the light source can be lessened. Consequently, the recording and reproduction performance of a signal becomes stable.
Preferably, the different diffraction order for the first and second laser light beams are respectively selected from the following combinations:
positive primary diffraction and positive secondary diffraction; and
negative secondary diffraction and negative primary diffraction.
Preferably, the light receiving method further comprises the step of passing the first and second laser light beams reflected from the optical recording medium through an enlarging optics, before or after the diffracting step.
If the beam diameter is thus enlarged, occurrence of a signal detection error for a light receiving position shift on the common light receiving element can be more suppressed.
Preferably, the light receiving method further comprises the step of detecting at least focusing errors in the first and second laser light beams, based on light amounts detected by the light receiving element. The focusing errors in the first and second laser light beams are detected by the same processing. In this configuration, signal processing circuit can be simplified.
An optical pickup device using the above described method comprises a diffractive element for diffracting the first and second laser light beams reflected from the optical recording medium based on different diffraction order, which is formed with a diffraction grating having a saw-toothed cross sectional shape.
In this configuration, the diffractive direction can be set to a predetermined direction and the yield of light can be enhanced.
Preferably, the diffractive element comprises a concave lens function.
The diffractive element can be provided with a concave lens function. In doing so, the beam diameter of return light can be enlarged for suppressing occurrence of a signal detection error caused by light receiving position change in the common light receiving element, and the function and effect can be provided without increasing the number of optical elements.